The dispersion of carbon black as a reinforcing filler in elastomer formulations is well-known for being highly influential in achieving optimal physical properties in rubber compounds. Methods to ensure the adequate dispersion of this critical ingredient include various chemical and physical approaches. The ongoing challenge for rubber compounders, mixers and processors is to adequately mix and distribute the reinforcing ingredient in a timely and efficient manner to achieve the maximum physical properties in the minimum amount of mixing time.
Early approaches used various soaps and waxes to wet out the reinforcing material and provide something for the polymer to grab onto during the initial stages of the rubber mixing process (see Takino et al., “Effect of Processing Additives on Carbon Black Dispersion and Grip Property of High Performance Tire Tread Compound,” Rubber Chemistry & Technology, March 1997, Vol. 70, No. 1, pp. 15-24). Optimal use levels were approximately 1 phr (parts per hundred rubber).
Recent approaches have used silane and coupling-agent chemistry for coating the various reinforcing fillers with a layer that has one end like the filler and the other end like the polymer. This has been particularly useful for silica-based reinforcements and to a lesser degree with carbon black reinforcements. Optimal use levels were approximately 2-3 phr.
In the years 1999-2001, attempts were made to pursue dual-phase filler systems using both carbon black and precipitated silicas. A series of papers were written about this approach. See, for example, Wang et al., “New Generation Carbon Silica Dual Phase Filler Part I. Characterization and Application to Passenger Tire,” Rubber Chemistry & Technology, May 2002, Vol. 75, No. 2, pp. 247-263; and Wang et al., “Carbon-Silica Dual Phase Filler, a New Generation Reinforcing Agent for Rubber: Part IX. Application to Truck Tire Tread Compound,” Rubber Chemistry & Technology, March 2001, Vol. 74, No. 1, pp. 124-127. Much discussion arose over the mechanisms of dispersion and the role and effectiveness of each of the reinforcing phases. The optimal use levels were in the 10-100 phr range.
In more recent years, examples of small amounts of dissimilar materials have been considered. Efforts were made with talc as a potential dispersion aid as recent as 2012 (Hershberger, “Effect of Talc on Carbon Black Dispersion,” Rubber World, February 2012). Additionally, as recent as 2014, efforts were underway to evaluate the synergistic effect of carbon black and the dual-phase filler system mentioned in the preceding paragraph (Xiong et al., “Synergistic Effect of Carbon Black and Carbon-Silica Dual Phase Filler in Natural Rubber Matrix”, Polymer Composites, August 2014, Vol. 35, Issue 8, pp. 1466-1472).
Improvements are still desired commercially to enhance dispersion of carbon black within polymer systems, as well as to improve static and dynamic physical properties of polymers (such as, but not limited to, natural or synthetic rubber) containing carbon black.